Photochemical Events Research Articles

Determination of critical low light limit and adaptive physiological and biochemical traits regulating growth and yield of mustard (Brassica juncea Coss.).

Growth and physio-biochemical traits under different incident solar light intensities (100, 67, 50 and 25%) were studied in mustard in a semi-arid agroclimate region of Central India. Our comprehensive studies revealed that incident solar light intensities below about 67% were highly detrimental in mustard for its growth and grain yield. Major factors that contributed to the differential responses under varying light intensities were identified which holds importance for better understanding of low light adaptability in an important oilseed crop like mustard. Biomass index (ratio of dry biomass to height) has been established and evaluated for the differential growth performance of the crop under different light intensities. Biomass index progressively declined from 0.48 (open sunlight) to 0.11 (25% sunlight). Physio-biochemical factors were identified that were playing major role in manifestation of the differential growth and grain yield. Mustard exhibited its low light adaptive trends through differential down-regulation in the rates of net CO2 assimilation (PN), stomatal conductance, transpiration, thylakoid electron transport rate (ETR) and leaf wax level. For example, PN decreased from 35.88 (open light) to 11.64µmolm-2s-1 (25% sunlight). Photochemical events showed critical impact as evidenced by decreased PSII quantum yield, photochemical quenching (qP) and higher non-photochemical quenching (qN) that were clearly associated with physiological efficiency of the plants under varying light intensities. Leaf wax level decreased from 1.69mgg-1 fresh weight (open light) to 0.96mgg-1 fresh weight (25% sunlight). Our results indicated that limited ETR supply across photosystem II (PSII) decreased the photochemical efficiency and carbon gain under low light which resulted in reduction of biomass index and grain yield. Besides, it was found that overexpression of protein band around ~ 26kDa in low light could be another adaptive feature for mustard related to light harvesting complex. Our findings would augment selection of traits for optimizing growth and grain yield of mustard for low light or light limiting agro-ecosystem.

Air pollution and control: Past, present and future

The quality of air is closely related to everyone’s life. However, due to the intense industrialization and urbanization, air pollution has affected or is affecting the health of the public. Since the notorious London smog and the Los Angeles photochemical smog events, the air pollution has been developing rapidly in recent years in some developing countries such as China and India. The essence of air pollution hazard is the impact of air pollutants on human health and environment. In view of the fragmentation of the research and one-sided knowledge of someone on air pollution and control, this paper is documented to review the whole status of technology development in this research field, to clarify the formation and elimination mechanism of air pollution, to correctly understand the hazard of air pollution and to promote the precise management of air pollution control. The research of the air pollution hazard is essentially the study of the effect of atmospheric pollution on human health and environment. The past, present and future status of air pollution and its control will be reviewed in this paper from the perspective of physical, chemical and biological process which are be modeling using mathematical technique, coding and programing. Meanwhile, the history, present and existing problems on atmospheric physics, atmospheric chemistry, emission inventory, atmospheric environment monitoring, meteorological field forecast, air quality forecast, source apportionment and tracing source, the impact of air pollution on human health, and atmospheric pollution control are well summarized. From the classical analytical methods developed in the middle of the last century such as the Gaussian method to the numerical methods generally used so far, the physical processes of transmission, diffusion and sedimentation of pollutant are described and solved. The chemical processes represented by the London smog incident mainly sulfur compounds, by Los Angeles photochemical smog mainly O3 and nitrogenous compounds, and by Beijing-Tianjin-Hebei fog-haze pollution mainly mixture of nitrogen- containing compounds and sulfur-based compounds have made breakthrough progresses, and the chemical mechanism are basically clear. A series of biological effects will be occurred when the pollutants are inhaled. The chemical composition of air pollutants will have oxidative stress reactions, lipid peroxidation and DNA damage, causing inflammation, cardiovascular disease, cancer and birth defects. At present, the research on the physical process is relatively mature, and the accuracy and effective days of weather forecast are increasing and extending gradually. However, the two-way feedback mechanism of meteorology and atmospheric pollution is still a difficult issue. The understanding of the main chemical processes is basically clear, but a large number of unknown reaction mechanisms need to be further explored, especially the chemical mechanism of the explosive growth of PM2.5. Cognition of biological processes is still preliminary, which are based primarily on statistical analysis but lack a theoretical basis and are needed to be further explored. The theoretical framework, as well as core issues of air pollution hazard identification and control, are be put forward and pointed out based on the theories, techniques, methods and standards of inversion and generation of emission sources, particle toxicity identification, chemical process assimilation, health risk prediction and early warning, synchronization source apportionment of predicting results, and dynamic optimal control are not yet mature, which are the main focus for future development, due to the extremely complicated and staggered influence as well as constraints on physical process, chemical process, biological process as well as mathematical description and computer calculation principle of air pollution and prevention. If the toxicity of pollutants is directly related to health risks, which may provide more direct, effective and scientific technological support for air pollution emergency optimal control as well as the optimal issues of industrial, energy structure adjustment and heavy pollution sources layout, it will play a fundamental role to meet the objective needs of identifying accurately pollution sources, achieving targeted management and promoting ecological civilization construction.

Charge Stabilization in High-Potential Zinc Porphyrin-Fullerene via Axial Ligation of Tetrathiafulvalene

Extending the lifetime of the charge-separated states generated during photoinduced electron transfer in a covalently linked high-potential zinc porphyrin-fullerene dyad, (F15P)Zn–C60, was accomplished by metal–ligand axial coordination of pyridine-functionalized tetrathiafulvalene (TTF) via a dual-electron-transfer/hole migration mechanism. The meso-aryl positions of the zinc porphyrin carried three penta-fluorophenyl substituents that made the zinc porphyrin ring harder to oxidize by 0.43 V compared with zinc porphyrin with meso-phenyl substituents. Two TTF derivatives, a first with a pyridine directly linked to TTF (Py-TTF) and a second with a phenyl spacer between the pyridine and TTF (Py-phTTF), were employed to vary the distance between the primary photosensitizer/electron donor, zinc porphyrin, and the secondary electron donor, TTF. Both Py-TTF and Py-phTTF coordinated via the pyridine entity to the Zn center with 1:1 molecular stoichiometry and moderate binding constants. The supramolecular triads were characterized by optical absorption and emission, electrochemistry, and computational studies. An energy-level diagram was established to realize the different photochemical events in the triads. Using femtosecond transient absorption spectroscopy, it was possible to show that the coordinated TTF participated in electron transfer from the 1(F15P)Zn* in the case of the C60-(F15P)Zn:TTF triads to produce C60-(F15P)Zn•-:TTF•+ charge-separated state competitively with the electron transfer from the 1(F15P)Zn* to covalently linked C60 to produce C60•--(F15P)Zn•+:TTF charge-separated state. The two charge-separated states, C60-(F15P)Zn•-:TTF•+ and C60•--(F15P)Zn•+:TTF, were further involved in electron migration in the former case and hole transfer in the latter case to produce the C60•--(F15P)Zn:TTF•+ charge-separated state as the ultimate electron-transfer product. Due to distal separation of the positive and negative radical ions, long-lived charge-separated states persistent for about 0.35 μs was possible to accomplish, as shown by nanosecond transient absorption spectral studies.

Surface Photovoltage Spectroscopy Resolves Interfacial Charge Separation Efficiencies in ZnO Dye-Sensitized Solar Cells

Any optimization of dye-sensitized solar cells (DSSCs) must consider the energetics and charge transfer kinetics of the dye, substrate, and redox couple. Here, we use surface photovoltage spectroscopy to probe the energetics and photochemical charge transfer efficiency in fluorenyl-thiophene dye (OD-8)-sensitized ZnO films. Discrete photochemical charge transfer events at the dye–ZnO interface and at the dye– I–/I3– or [Co(2,2′-bipyridyl)3]3+/2+ interfaces can be observed as negative photovoltage under dye excitation at 1.7 eV (460 nm). Without a redox couple, charge separation at the ZnO/dye interface is only 4% effective, likely due to the short electron hole separation distance. In the presence of the redox couples, charge separation approaches 26–54% of the theoretical limit, emphasizing the importance of the dye regeneration reaction via the redox couple. On the basis of the open circuit voltage, charge separation in fully assembled DSSCs is 100% efficient with iodide, but only 61% efficient with the...

Seasonal Source Apportionment of PM2.5 in Ningbo, a Coastal City in Southeast China

With the rapid economic development and urbanization of China, haze and photochemical smog events have been frequently observed during the last decade. To explore the temporal and spatial pollution characteristics in Ningbo, a medium-sized coastal city located in the Yangtze River Delta (YRD) in southeast China, 24-h PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 µm) samples were simultaneously collected at five sites (two urban residential sites, two urban coastal sites, and one suburban site) from winter 2012 to autumn 2013. The average PM2.5 concentration was 53.2 ± 30.4 µg m–3. Furthermore, the concentration exhibited a seasonal variation: It was highest in winter and lowest in summer. The urban residential sites had the highest PM2.5 concentrations, followed by the urban coastal sites, and the suburban site had the lowest concentration. OM (Organic Matters) and secondary inorganic ions (sulfate, nitrate, and ammonium) were the dominant components of the PM2.5. As a coastal city with industrial zones, sources are more complex in Ningbo than in inland cities due to ship emissions and the interactions between land and sea, and the marine and atmospheric environments. Positive matrix factorization (PMF) was used to apportion the particle sources. Nine factors were resolved in this study: secondary nitrate, vehicle exhaust, secondary sulfate, coal combustion, industrial emission, ship emission, dust, biomass burning, and aged sea salt, with average contributions of 26%, 21%, 13%, 12%, 9%, 7%, 5%, 4%, and 3%, respectively. Secondary nitrate and vehicle exhaust were the major sources of PM2.5 pollution in Ningbo. Coal combustion contributed significantly in winter and autumn, whereas sea salt formed a considerable contribution in summer. This study suggests that decreasing the PM2.5 pollution in Ningbo requires not only strategies for reducing local primary sources but also joint inter-regional prevention and the control of air pollution in the YRD.

The sacrificial inactivation of the blue-light photosensor cryptochrome from Drosophila melanogaster.

Drosophila melanogaster cryptochrome functions as the primary blue-light receptor that mediates circadian photo entrainment. Absorption of a photon leads to reduction of the protein-bound FAD via consecutive electron transfer along a conserved tryptophan tetrad resembling the signalling state required for conformational changes and induction of subsequent signalling cascades. However, how the initial photochemistry and subsequent dark processes leading to downstream signalling are linked to each other at the molecular level is still poorly understood. Here, we investigated in detail the initial photochemical events in DmCRY by time-resolved and stationary absorption spectroscopy combined with quantum chemical and molecular dynamics calculations. We resolved the early events along the conserved tryptophan tetrad and the final deprotonation of the terminal tryptophanyl radical cation. These initial events lead to conformational changes, such as the known C-terminal tail release, Trp decomposition, and finally FAD release providing evidence that DmCRY does not undergo a photocycle. We propose that light is a negative regulator of DmCRY stability even under in vitro conditions where the proteasomal machinery is missing, that is in line with its biological function, i.e. entrainment of the circadian clock.

Photoinduced energy transfer in carbazole-BODIPY dyads.

A series of carbazole (CBZ)-boron dipyrromethene (BODIPY) based donor-acceptor dyads, CB1, CB2, and CB3, with CBZ as an energy donor, tethered together with spacers of varied sizes i.e., phenyl bridge, biphenyl bridge and diphenylethyne bridge, respectively, are reported. The newly synthesized dyads were characterized by various spectroscopic techniques. A comparison of the absorption and electrochemical data of the dyads with their reference compounds (i.e., 9-phenyl-9H-carbazole (C0) and N,N'-difluoroboryl-1,3,7,9-tetramethyl-5-phenyldipyrrin (B0)) revealed minimal ground-state interactions between the chromophores. Selective excitation of CBZ in the dyads at 290 nm resulted in the quenching of the CBZ emission followed by the appearance of BODIPY emission, revealing efficient energy transfer from singlet excited CBZ (1CBZ*) to BODIPY. The photoinduced energy transfer phenomenon was studied in three different solvents of varying polarity. The driving forces for energy transfer (ΔGEN) for all the dyads were found to be exothermic. The rate constants for energy transfer, kENT, measured by the femtosecond transient absorption technique in toluene were found to be in the range of 0.8-2.0 × 1010 s-1, depending on the type of spacer between the CBZ and BODIPY entities, and were in close agreement with the theoretically estimated rates according to the Förster energy transfer model. In contrast, selective excitation of BODIPY in these dyads at 485 nm resulted in small quenching of the BODIPY emission, suggesting a lack of major photochemical events originating from 1BODIPY*.

Resolving the Notorious Case of Conical Intersections for Coupled Cluster Dynamics.

The motion of electrons and nuclei in photochemical events often involves conical intersections, or degeneracies between electronic states. They serve as funnels in nuclear relaxation processes where the electrons and nuclei couple nonadiabatically. Accurate ab initio quantum chemical models are essential for interpreting experimental measurements of such phenomena. In this Letter, we resolve a long-standing problem in coupled cluster theory, presenting the first formulation of the theory that correctly describes conical intersections between excited electronic states of the same symmetry. This new development demonstrates that the highly accurate coupled cluster theory can be applied to describe dynamics on excited electronic states involving conical intersections.

Taming Macromolecules with Light: Lessons Learned from Vibrational Spectroscopy.

Exciting new applications, from large-area nanopatterning and templating to soft light-powered robotics, are emerging from the fundamental research on light-triggered changes in macromolecular systems upon photoisomerization of azobenzene-based molecular photoswitches. The understanding of how the initial molecular-scale photoisomerization of azobenzene, a complex photochemical event in itself, is translated into the response of macromolecules and even into macroscopic-scale motion of illuminated azomaterials is an enormous task. The focus here is on how this knowledge has advanced by applying different vibrational spectroscopy techniques that provide rich molecular insight into the photoresponse of chemically specific molecular moieties. In particular, infrared and Raman spectroscopy studies are highlighted, in the context of phototriggered perturbation of self-assembled structures and photoinduced linear and circular anisotropy, as well as photoinduced surface patterning, with the objective of offering a perspective on how vibrational spectroscopy can help in answering an array of essential yet unsettled questions.

Chromophore twisting in the excited state of a photoswitchable fluorescent protein captured by time-resolved serial femtosecond crystallography.

Chromophores absorb light in photosensitive proteins and thereby initiate fundamental biological processes such as photosynthesis, vision and biofluorescence. An important goal in their understanding is the provision of detailed structural descriptions of the ultrafast photochemical events that they undergo, in particular of the excited states that connect chemistry to biological function. Here we report on the structures of two excited states in the reversibly photoswitchable fluorescent protein rsEGFP2. We populated the states through femtosecond illumination of rsEGFP2 in its non-fluorescent off state and observed their build-up (within less than one picosecond) and decay (on the several picosecond timescale). Using an X-ray free-electron laser, we performed picosecond time-resolved crystallography and show that the hydroxybenzylidene imidazolinone chromophore in one of the excited states assumes a near-canonical twisted configuration halfway between the trans and cis isomers. This is in line with excited-state quantum mechanics/molecular mechanics and classical molecular dynamics simulations. Our new understanding of the structure around the twisted chromophore enabled the design of a mutant that displays a twofold increase in its off-to-on photoswitching quantum yield.

Competitive Energy and Electron Transfer in β-Functionalized Free-Base Porphyrin-Zinc Porphyrin Dimer Axially Coordinated to C60 : Synthesis, Supramolecular Formation and Excited-State Processes.

Simultaneous occurrence of energy and electron transfer events involving different acceptor sites in a newly assembled supramolecular triad comprised of covalently linked free-base porphyrin-zinc porphyrin dyad, H2 P-ZnP axially coordinated to electron acceptor fullerene, has been successfully demonstrated. The dyad was connected through the β-pyrrole positions of the porphyrin macrocycle instead of the traditionally used meso-positions for better electronic communication. Interestingly, the β-pyrrole functionalization modulated the optical properties to such an extent that it was possible to almost exclusively excite the zinc porphyrin entity in the supramolecular triad. The measured binding constant for the complex with 1:1 molecular stoichiometry was in the order of 104 m-1 revealing moderately stable complex formation. An energy level diagram constructed using optical, electrochemical and computational results suggested that both the anticipated energy and electron events are thermodynamically feasible in the triad. Consequently, it was possible to demonstrate occurrence of excited state energy transfer to the covalently linked H2 P, and electron transfer to the coordinated ImC60 from studies involving steady-state and time-resolved emission, and femto- and nanosecond transient absorption studies. The estimated energy transfer was around 67 % in the dyad with a rate constant of 1.1×109 s-1 . In the supramolecular triad, the charge separated state was rather long-lived although it was difficult to arrive the exact lifetime of charge separated state from nanosecond transient spectral studies due to overlap of strong triplet excited signals of porphyrin in the monitoring wavelength window. Nevertheless, simultaneous occurrence of energy and electron transfer in the appropriately positioned energy and electron acceptor entities in a supramolecular triad was possible to demonstrate in the present study, a step forward to unraveling the complex photochemical events occurring in natural photosynthesis and its implications in building light energy harvesting devices.

Flexible interlocked porous frameworks allow quantitative photoisomerization in a crystalline solid

Photochromic molecules have shown much promise as molecular components of stimuli-responsive materials, but despite recent achievements in various photoresponsive materials, quantitative conversion in photochemical reactions in solids is hampered by the lack of intrinsic structural flexibility available to release stress and strain upon photochemical events. This issue remains one of the challenges in developing solid-state photoresponsive materials. Here, we report a strategy to realize photoresponsive crystalline materials showing quantitative reversible photochemical reactions upon ultraviolet and visible light irradiation by introducing structural flexibility into crystalline porous frameworks with a twofold interpenetration composed of a diarylethene-based ligand. The structural flexibility of the porous framework enables highly efficient photochemical electrocyclization in a single-crystal-to-single-crystal manner. CO2 sorption on the porous crystal at 195 K is reversibly modulated by light irradiation, and coincident X-ray powder diffraction/sorption measurements clearly demonstrate the flexible nature of the twofold interpenetrated frameworks.

Photochemical Kinetics of a Phosphine Oxide Free Radical Initiator from Femtosecond UV-Pump/Mid-IR-Probe Spectroscopy.

Femtosecond UV-pump/mid-infrared-probe spectroscopy was used to explore in detail the primary photochemical events of the free radical initiator, (2,4,6-trimethylbenzoyl)diphenylphosphine oxide, in liquid dichloromethane solution at room temperature. Following electronic excitation of its lowest excited singlet state, S1, the radical initiator undergoes an intersystem crossing to the triplet ground state, T1, with a time constant of 135 ps. A subsequent α-cleavage occurs from the triplet state with a time constant of 15 ps and yields a trimethylbenzoyl radical together with a diphenylphosphinoyl radical. Transient absorptions from the S1 and T1 states are observed that can be assigned to the P═O stretching mode and the symmetric in-plane deformation mode of the trimethylphenyl moiety of the radical initiator.

Addition of Hydrogen Peroxide to Groundwater with Natural Iron Induces Water Disinfection by Photo-Fenton at Circumneutral pH and other Photochemical Events.

Samples of natural groundwater (with low turbidity, neutral pH and 0.3 mg L-1 iron concentration) inoculated with Escherichia coli K-12 were exposed to simulated solar light both in the presence and in the absence 10 mg L-1 of H2 O2. Results demonstrated that the viability of E. coli (by DVC-FISH) was grounded to zero after 360 min of irradiation. This abatement could be caused by the oxidative stress induced by ·OH radicals or another photo-induced reactive oxygen species. Two 23 factorial experimental designs enabled the evaluation of the effects of chemical factors on the inactivation of E. coli. The first experimental design considered the pH, iron and H2 O2 , while the second evaluated the ions fluoride, carbonate and phosphate found in groundwater. pH was found to play a key role in the inactivation of E. coli. The best reduction in viability was obtained at the lower pH (6.75), while a nonsignificant effect was observed when iron or H2 O2 concentrations were raised. At higher concentrations, anions, such as carbonate and phosphate, negatively affected the E. coli abatement. However, a higher concentration of fluoride accelerated it. In all experiments, the pH was observed to rise to values higher than 8.0 units after 360 min of treatment.

(Invited) Time-Resolved EPR Study on Charge Dynamics of Electron-Hole Pairs in Lead Iodide Perovskite Thin Film

For developments of the light-energy conversion systems, it is essential to achieve efficient long-range charge-separations (CS). Light-induced electrons and holes generated by the electron-transfer (ET) reactions are thus required to escape from the electrostatic binding at the initial stage. In this respect, the lead iodide perovskite solar cells have attracted great attentions since they exhibit quite efficient energy conversion performances. This solar cell adopts the hybrid metal iodide perovskites as a photoactive layer with the general formulation of ABX3(CH3NH3PbI3), where A is the methylammonium (CH3NH3 +) cation, B is Pb2+ and M is I-. In the devise structure, this photoactive layer is sandwiched by the compact titanium oxide (TiO2) layer and by the hole-transporting layer to efficiently collect the photocarriers generated by the light irradiations of the perovskite, giving rise to ~20% power conversion efficiencies (PCE). Recent studies have clearly shown that the carrier mobilities of the electrons and the holes are significantly high in the CH3NH3PbI3 layer, resulting in micrometer ranges of the charge-diffusion lengths.[1] These efficient charge diffusions play a crucial role on the prevention of the unwanted geminate charge-recombination in the electron-hole pairs. However, little is known concerning the electronic interactions of the electron-hole pairs initially generated in the CH3NH3PbI3 layer. At lower temperature (T < 160 K), it is reported that the electron and the hole generated by the light irradiation exhibit a Wannier exciton character with a biding energy of ~30 meV.[2] Thus, low temperature investigations of the electronic characters in the electron-hole pairs are very important to understand the mechanisms of the efficient charge-conductions initiated by the primary electron-hole pairs in the CH3NH3PbI3 layer. Another importance is to understand electronic characters of the trapped-charges generated by the light irradiations of the photoactive layers. It is recently reported that the charge careers are trapped at the surface area of the crystalline CH3NH3PbI3 layer.[3] However, electronic orbital characteristics of the trapped charges are not well known in the devices. Time-resolved electron paramagnetic resonance (TREPR) and pulsed EPR methods have been powerful to obtain the electron spin-spin interactions including the spin-spin exchange coupling of the photoinduced CS states and thus have been utilized to characterize the interspin distances and the electronic couplings of the transient CS states.[4] Additionally, TREPR can detect the electron spin polarization (ESP) as an effect of the initial photochemical events. This phenomenon is referred to as the chemically induced dynamic electron polarization (CIDEP). According to the radical pair mechanism (RPM) CIDEP, the ESP is generated as a result of the coherent singlet-triplet interconversion in the presence of the exchange coupling (J) of the primary radical-pair states, producing the enhanced microwave absorption and the microwave emission in the separated radicals during the diffusive separations of the individual radicals in the radical-pair.[5] Since the ESP detected as the microwave absorption (A) or the emission (E) is thus sensitive to J, one is able to access the electronic interactions in the primary radical-pairs.[6] To unveil the above fundamental electronic characteristics for the light-induced charge conductions, we have observed low temperature TREPR spectra for the three layer thin films (Spiro-OMeTAD/CH3NH3PbI3/TiO2 on a glass substrate) composed of the organic hole transport layer (Spiro-OMeTAD), the CH3NH3PbI3 layer and the mesoporous TiO2 fabricated by spin coating method on the mesoporous TiO2 films. From the EPR spectra, we have characterized the spin density distributions of the trapped holes of the CH3NH3PbI3 layer at 110 K from the anisotropic hyperfine structures originating from the iodine atoms (I). Furthermore, from the ESP generated in the trapped hole, we estimated the J coupling in the primary electron-hole pairs which will give us an insight on the electronic coupling of the Wannier excitons. 1.﷒ Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Science 2013, 342, 344. 2.﷒ Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. J. Am. Chem. Soc. 2014, 136, 11610-11613. 3.﷒ Tachikawa, T.; Karimata, I.; Kobori, Y. J. Phys. Chem. Lett. 2015, 6, 3195-3201. 4. Kobori, Y.; Miura, T. J. Phys. Chem. Lett. 2015, 6, 113-123. 5.﷒ Kobori, Y.; Yago, T.; Akiyama, K.; Tero-Kubota, S.; Sato, H.; Hirata, F.; Norris, J. R. J. Phys. Chem. B 2004, 108, 10226-10240. 6. Kobori, Y.; Yamauchi, S.; Akiyama, K.; Tero-Kubota, S.; Imahori, H.; Fukuzumi, S.; Norris, J. R. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10017-10022.

A study of the exchangeable protons in the QA site of photosystem II

Plants have developed the only biological system capable of oxidizing water. Through the oxidation of water plants are able to convert light energy into chemical energy. This water oxidation is catalyzed by photosystem II (PSII) at a site called the oxygen‐evolving complex (OEC). The OEC consists of a tetranuclear manganese cluster and is the only known biological system capable of oxidizing water (Blakenship 2001).Removal of the manganese cluster allows for study of the mechanism called photoactivation, a light‐and dark‐dependent process by which the OEC assembles. Further study of this process can provide great detail as to repair of PSII following inhibition (G. Dismukes 2008). Removal of the manganese cluster and associated extrinsic proteins also allows unhindered exploration of electron transfer during photochemical events. Plastoquinone A (QA) and plastoquinone B (QB) are electron carriers found in PSII. Despite sharing the same molecular structure, QA and QB exhibit unique redox properties determined by their protein environment. QA participates only in one‐electron transfers, transiently forming paramagnetic semiquinone QA−•(SQA), whereas QB function as a two‐electron gate. Hydrogen bonding from nearby amino acids residues is believed to be partly responsible for such differences in redox properties due to redistribution of electron density. High‐resolution electron paramagnetic resonance (EPR) spectroscopy has the ability to measure magnetic interactions between unpaired electrons and nearby nuclei and provide information about the strength of these interactions (Rutherford, A. W. 1999).Previous pulsed EPR studies established the formation of the H‐bonds between carbonyl oxygens of the QA−• and an amide nitrogen from the protein backbone, and the amino nitrogen, Nd, of an imidazole (Rutherford; Lakshmi). Two‐dimensional pulsed EPR of the SQA in Mn depleted, CN‐treated PSII at pH 9.8, resolved five pairs of cross‐peaks from different types of protons (Lakshmi). They were assigned to the methyl, β‐methylene, and ring protons of the SQA and two protons of the H‐bonds. Unfortunately, these assignments were not verified by the influence of 1H2O/2H2O exchange on the spectra. As a consequence, the assignment of the couplings is not consistent with the values obtained in the DFT calculations (T.‐J. Lin and P. J. O'Malley 2011). In this work we performed 2D pulsed EPR with the SQA in the PS II samples in 1H2O and 2H2O buffered solution in order to unambiguously identify contribution of the exchangeable protons and to determine corresponding hyperfine couplings. These results form a more supported basis for comparison with the results of theoretical computations.Support or Funding InformationPulsed EPR studies were supported by DE‐FG02‐08ER15960 Grant from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, US DOE (S.A.D.), and NCRR/NIH Grants S10‐RR15878 and S10‐RR025438 for pulsed EPR instrumentation. Continuous wave EPR studies were supported by NSF‐MRI CHE‐1229498.

Photoluminescence Intermittency and Photo-Bleaching of Single Colloidal Quantum Dot.

Photoluminescence (PL) blinking of single colloidal quantum dot (QD)-PL intensity switching between different brightness states under constant excitation-and photo-bleaching are roadblocks for most applications of QDs. This progress report shall treat PL blinking and photo-bleaching both as photochemical events, namely, PL blinking as reversible and photo-bleaching being irreversible ones. Most studies on single-molecule spectroscopy of QDs in literature are related to PL blinking, which invites us to concentrate our discussions on the PL blinking, including its brief history in 20 years, analysis methods, competitive mechanisms and different strategies to battle it. In terms of suppression of the PL blinking, wavefunction confinement-confining photo-generated electron and hole within the core and inner portion of the shell of a core/shell QD-demonstrates significant advantages. This strategy yields nearly non-blinking QDs with their emission peaks covering most part of the visible window. As expected, the resulting QDs from this new strategy also show substantially improved anti-bleaching features.

Reactivity III: An Advanced Course in Integrated Organic, Inorganic, and Biochemistry

Reactivity III is a new course that presents chemical reactions from the domains of organic, inorganic, and biochemistry that are not readily categorized by electrophile–nucleophile interactions. Many of these reactions involve the transfer of a single electron, in either an intermolecular fashion in the case of oxidation/reduction reactions or an intramolecular fashion in the case of photochemical events. Others, such as pericyclic reactions, involve movement of electron pairs but lack obvious acid/base analogies and build on an understanding of photochemistry.

Geometry‐Controlled Photoinduced Charge Separation and Recombination in a Trans‐A2B2‐Functionalized Donor–Acceptor Conjugate Composed of a Multimodular Zinc Porphyrin and Fullerene

AbstractThe front cover artwork is provided by the D′Souza group at the Department of Chemistry, University of North Texas. The image shows the optimized structure of a trans‐A2B2‐functionalized multimodular zinc porphyrin/fullerene supramolecular donor–acceptor conjugate and the geometry‐governed photochemical events which occur upon photoexcitation of the porphyrin. Read the full text of the article at 10.1002/cptc.201600017.

Controlling electron and energy transfer paths by selective excitation in a zinc porphyrin-BODIPY-C60 multi-modular triad.

A multi-modular donor-acceptor triad composed of zinc porphyrin, BF2-chelated dipyrromethene (BODIPY), and C60 was newly synthesized, with the BODIPY entity at the central position. Using absorbance and emission spectral, electrochemical redox, and computational optimization results, energy level diagrams for the ZnP-BODIPY dyad and ZnP-BODIPY-C60 triad were constructed to envision the different photochemical events upon selective excitation of the BODIPY and ZnP entities. By transient absorption spectral studies covering a wide femtosecond-to-millisecond time scale, evidence for the different photochemical events and their kinetic information was secured. Efficient singlet-singlet energy transfer from 1BODIPY* to ZnP with a rate constant kENT = 1.7 × 1010 s-1 in toluene was observed in the case of the ZnP-BODIPY dyad. Interestingly, in the case of the ZnP-BODIPY-C60 triad, the selective excitation of ZnP resulted in electron transfer leading to the formation of the ZnP˙+-BODIPY-C60˙- charge-separated state. Owing to the distal separation of the radical cation and radical anion species (edge-to-edge distance of 18.7 Å), the radical ion-pair persisted for microseconds. By contrast, the selective excitation of BODIPY resulted in an ultrafast energy transfer to yield ZnP-BODIPY-1C60* as the major product. The 1C60* populated the low-lying 3C60* via intersystem crossing prior to returning to the ground state. The present study successfully demonstrates the importance of supramolecular geometry and selection of excitation wavelength in regulating the different photoprocesses.